Catalytic converters may be present in the exhaust stream of motor vehicles in order to reduce the emission of pollutants produced during combustion. For example, unburnt fuel and carbon monoxide (CO) present in the exhaust released by an internal combustion engine may be converted to less toxic substances, such as carbon dioxide and water, by the catalytic converter before being released to the atmosphere. The catalytic converters function efficiently and optimally at a certain operating temperature. The catalytic converters do not each the operating temperature (also known as a light-off temperature) immediately after the start of the internal-combustion engine, and hence, the internal-combustion engine exhaust may be released into the environment without complete conversion of the unburnt hydrocarbons, thereby degrading emissions.
Other attempts to address the problem of emission degradation during cold start of an engine due to low catalytic converter temperature include storing the exhaust in a tank until the catalytic converter reaches the light-off temperature after which the stored exhaust may be directed through the catalytic converter and released to atmosphere, as shown in U.S. Pat. No. 6,250,073.
The inventors herein have recognized an issue with the above approach. In addition to being operated at or above light-off temperature, catalytic converters also achieve optimal conversion rates when the air-fuel ratio of exhaust entering the catalytic converter is controlled to a certain air-fuel ratio, such as stoichiometry. Further, besides cold start conditions, the exhaust gas will have a high load of unburnt hydrocarbons during other engine operating conditions, such as during a transient boost demand. The engine may be operated with richer fuel to meet the transient boost demand, for example, during acceleration, which deviates the air fuel ratio from stoichiometry, resulting in degradation of emissions even though the catalytic converter is at the target operating temperature.
In one example, the issues described above may be addressed by a method for operating an exhaust system of an engine, including directing a flow of exhaust gas from downstream of a catalytic converter to a tank in response to engine air-fuel ratio deviating from a threshold air-fuel ratio, adjusting an air-fuel ratio of the exhaust gas in the tank, and selectively releasing the exhaust gas from the tank to upstream of the catalytic converter, the released exhaust gas adjusted to the threshold air-fuel ratio.
In one example, directing exhaust flow from downstream of the catalytic converter to the tank may be performed in response to an air fuel ratio of the tank being off-stoichiometry. In a further example, directing exhaust flow from downstream of the catalytic converter to the tank may be performed in response to a degraded oxygen sensor upstream of the catalytic converter. In another example, directing exhaust flow from downstream of the catalytic converter to the tank may be performed in response to a temperature of the catalytic converter being below a threshold temperature. The releasing of exhaust from the tank may be performed in response to a pressure inside the tank being more than a minimum pressure in combination with the temperature of the catalytic converter being more than the threshold temperature and an exhaust air-fuel ratio stored in the tank being stoichiometric.
In this way, exhaust may be stored in a tank coupled to a catalytic converter both during cold start conditions and during conditions where an air fuel ratio of the engine is off-stoichiometry. The stored exhaust gas may be re-circulated back through the catalytic converter when the catalytic converter is capable of efficiently breaking down the hydrocarbons in the exhaust gas before being released to atmosphere, thus reducing degradation of tail pipe emissions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.